The present invention relates to a process for selectively producing organic trisulfides.
Organic polysulfides, especially organic trisulfides, are useful for many purposes such as additives for elastomers, antioxidants for lubricating oils, intermediates for the production of organic chemicals, insecticides, and germicides, and additives for diesel fuels to improve cetane number and ignition qualities. Organic polysulfides are also useful in the compounding of high pressure lubricants and in the acceleration of rubber treating processes.
It is known that organic polysulfides can be produced by reacting mercaptans with elemental sulfur in the presence of a catalyst. The product of such reaction typically comprises a distribution of individual organic polysulfide compositions, each containing a different number of sulfur atoms. In many commercial applications, especially for use in high pressure lubricants, organic trisulfides exhibit more desirable properties than other organic polysulfides. For example, organic polysulfides containing more than three sulfur atoms exhibit high copper-strip corrosivity (ASTM Copper Strip Corrosion Test D-130-56), rendering them unsatisfactory for many commercial applications. In addition, organic disulfides can be undesirable because they have a high flash point and exhibit poor lubrication properties.
If an organic polysulfide product contains a high quantity of organic polysulfides having more or less than three sulfur atoms, costly separation processes and equipment can be necessary to remove undesirable polysulfides and recover a more pure organic trisulfide product that is suitable for commercial purposes. Therefore, a reaction product having a distribution of organic polysulfides which maximizes the amount of organic trisulfides while minimizing the amount of other organic polysulfides is desired.
Conventional processes for making high-purity organic trisulfides typically require expensive catalysts and multiple reaction vessels, both of which contribute to the expense of the overall process.
In addition, conventional processes for making high-purity organic trisulfides typically require the mercaptan-sulfur reaction to be terminated before substantial completion in order to avoid the formation of undesirable organic polysulfides in the reaction product. Terminating the mercaptan/sulfur reaction before it goes to substantial completion causes the presence of unreacted mercaptan and/or sulfur in the reaction product. Such unreacted mercaptan and/or sulfur must be removed from the reaction product, thus requiring additional costly separation processes and equipment.
An object of the present invention is to provide a process for selectively producing organic trisulfides.
Another object of the inventive process is to provide an organic polysulfide product having a higher concentration of organic trisulfides than products from conventional processes for making organic polysulfides.
A further object of the inventive process is to provide an organic polysulfide product having a lower concentration of organic tetrasulfides than products from conventional processes for making organic polysulfides.
A still further object of the inventive process is to provide an organic polysulfide product having a lower concentration of organic disulfides than products from conventional processes for making organic polysulfides.
An even further object of the present invention is to eliminate the need for expensive processes and equipment necessary to separate and recover a substantially pure organic trisulfide product.
A still further object of the inventive process is to eliminate the need for expensive catalysts which are required by conventional methods of selectively producing organic trisulfides.
Other objects and advantages of the present invention will become more apparent as the invention is more fully disclosed hereinbelow.
According to an embodiment of the present invention, a process for the selective production of organic trisulfides is provided. The process comprises contacting a mercaptan, a sulfur compound, and a catalyst in a reaction vessel under reaction conditions sufficient to produce a reaction product comprising an organic polysulfide product and subjecting the reaction product to trisulfide enhancing conditions which are effective to inhibit the formation of organic disulfides and promote the conversion of organic tetrasulfides to organic trisulfides.
In another embodiment of the present invention a process for selectively producing organic trisulfides is provided. The process comprises (a) contacting a mercaptan, a sulfur compound, and a catalyst in a reaction vessel under reaction conditions sufficient to produce a reaction product comprising an organic polysulfide product and hydrogen sulfide, thereby creating within the reaction vessel a liquid reaction solution comprising a mixture of liquid phase compounds, wherein a liquid hydrogen sulfide concentration is present in the liquid reaction solution; (b) increasing, for an initial reaction period, the liquid hydrogen sulfide concentration to a disulfide inhibiting hydrogen sulfide concentration that is effective to inhibit the formation of organic disulfides within the liquid reaction solution; (c) maintaining the disulfide inhibiting hydrogen sulfide concentration for a final reaction period; (d) lowering the temperature and pressure in the reaction vessel to trisulfide enhancing conditions that are effective to inhibit the formation of organic disulfides and promote the conversion of organic tetrasulfides to organic trisulfides; and (e) maintaining the trisulfide enhancing conditions within the reaction vessel for a trisulfide enhancing period.
It has been discovered that, in a process for making organic polysulfides by reacting a mercaptan and a sulfur compound, the relative amount of organic trisulfides to other organic polysulfides in the reaction product can be increased by subjecting the product of the mercaptan/sulfuir reaction to trisulfide enhancing conditions.
The process of the present invention can be commenced by contacting a mercaptan, a sulfur compound, and a catalyst in a suitable reaction vessel to create a xe2x80x9cliquid reaction solutionxe2x80x9d in the reaction vessel. The contacting of the mercaptan, sulfur compound and catalyst is generally accomplished by slowly adding one of the reactants to a mixture of the other reactant and the catalyst.
As used herein, the term xe2x80x9cliquid reaction solutionxe2x80x9d means a mixture of all liquid phase compounds present in a reaction vessel. The composition of the liquid reaction solution of the present invention changes as the process progresses, however, during at least a portion of the process the liquid reaction solution can comprise unreacted mercaptan, the catalyst, organic polysulfides, and hydrogen sulfide.
The mercaptan suitable for use as a reactant in the process of the present invention can be any mercaptan having the formula of RSH, wherein R is a hydrocarbyl radical having 1 to about 30, preferably about 1 to about 20, and most preferably 2 to 15 carbon atoms. The hydrocarbyl radical can be linear or branched and can be alkyl, aryl, cycloalkyl, alkylaryl, aralkyl, alkenyl radicals, or combinations of two or more thereof. Preferably the hydrocarbyl radical is an alkyl radical. Presently preferred mercaptans are tertiary mercaptans. The presently most preferred mercaptan is t-butyl mercaptan.
The sulfur compound suitable for use as a reactant in the process of the present invention can be any sulfur containing compound capable of reacting with a mercaptan to produce an organic trisulfide and hydrogen sulfide. Preferably the sulfur compound is elemental sulfur.
The amount of sulfur compound contacted with the mercaptan depends on the desired sulfur content of the organic polysulfide product. For an average sulfur content of q sulfurs per polysulfide molecule, (q-1) moles of sulfur must be added per 2 moles of mercaptan and 1 mole of hydrogen sulfide will be produced per 2 moles of mercaptans reacted. It is, however, preferred that about 0.5 to about 10, preferably about 1.0 to about 5, and most preferably 1.0 to 2.0 moles of mercaptan per mole of sulfur is used.
The catalyst suitable for use in the process of the present invention can be any catalyst capable of catalyzing the reaction of a mercaptan and a sulfur compound to form hydrogen sulfide and an organic trisulfide. The presently preferred catalyst comprises a basic catalyst which can be an inorganic base, an organic base, or combinations of two or more thereof. Suitable organic bases include, but are not limited to tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetramethylammonium bisulfide, tetraethylammonium bisulfide, trimethylamine, triethylamine, n-butylamine and combinations of two or more thereof. Suitable inorganic bases include, but are not limited to, lithium hydroxide, sodium hydroxide, sodium bisulfide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, sodium bicarbonate, sodium carbonate, sodium oxide, sodium sulfide, magnesium oxide, calcium oxide, calcium carbonate, sodium phenoxide, barium phenoxide, calcium phenoxide, R1ONa, R1SNa, and combinations of any two or more thereof; where R1 is a C1-C18 alkyl radical, or combinations of any two or more thereof. Presently, the amine catalysts are not as preferred as other catalysts, and an inorganic base is preferred because of the availability and low cost of inorganic bases. Among the inorganic bases, sodium hydroxide is preferred because it is readily available and inexpensive.
The catalyst useful in the process of the present invention can further comprise an alkoxylated compound, preferably and alkoxylated alcohol. The alkoxylated alcohol useful in the present invention has a general formula of R2O[CH2CH(R3)O]mH where R2 is a C1-C20 hydrocarbyl radical selected from the group consisting of alkyl radical, alkylaryl radical, aryl radical, cycloalkyl radical and alkenyl radical. Preferably R2 is a C6-C18 alkyl radical. Most preferably R2 is a C10-C16 alkyl radical. Preferably R3 is selected from the group consisting of hydrogen, C1-C6 alkyl radicals, and C2-C16 alkenyl radicals. Preferably R3 is a hydrogen or a C1-C3 alkyl radical. Most preferably R3 is hydrogen. Preferably m is a number from 1 to about 20, more preferably from about 2 to about 12, and most preferably from 5 to 10. An example of a suitable alkoxylated alcohol is TERGITOL(copyright) 15-S-7 which is an ethoxylated alcohol, manufactured and marketed by Union Carbide Corporation, and having the formula of R2O(CH2 CH2O)7H where R2 is a secondary alkyl radical having 11-15 carbon atoms and 7 is the average number of the ethylene oxide units. Another example is an ethoxylated phenol having the same number of ethylene oxide units. Other suitable alkoxylated alcohols are also available from Union Carbide Corporation.
The weight ratio of base to alkoxylated compound in the catalyst can vary widely so long as the ratio can catalyze the reaction of a mercaptan and a sulfur compound. Preferably the weight ratio of a base to an alkoxylated compound is from about 10:1 to about 1:100, more preferably from about 2:1 to about 1:10, most preferably from 1:1 to 1:5.
The amount of catalyst contacted with the mercaptan and sulfur compound of the present invention is any amount that can catalyze the formation of an organic polysulfide. The weight of the catalyst as a percentage of the weight of mercaptans can be in the range of from about 0.001 to about 10 percent, preferably from about 0.01 to about 3 percent, and most preferably from 0.05 to 2 percent.
The organic polysulfides produced by the reaction of a mercaptan and a sulfur compound can be any organic polysulfides having the formula of RSxR, wherein each R can be the same or different and are each a hydrocarbyl radical having 1 to about 30, preferably 1 to about 20, and most preferably 2 to about 15 carbon atoms and x is a number from 2 to about 10, preferably 2 to 6, more preferably 3 to 5, and most preferably 3. The hydrocarbyl radical can be linear or branched and can be alkyl, aryl, cycloalkyl, alkylaryl, aralkyl, alkenyl radicals, or combinations of two or more thereof. Preferably the hydrocarbyl radical is an alkyl radical. Preferred organic polysulfides are di-t-butyl polysulfides. The most preferred organic polysulfide is di-t-butyl trisulfide.
In accordance with an embodiment of the present invention, during a reaction period, reaction conditions are employed which facilitate the reaction of the mercaptan and sulfur compound. The reaction period can commence with the commencement of the mercaptan/sulfur reaction and terminate when the mercaptan/sulfur reaction is substantially complete. The mercaptan/sulfur reaction is substantially complete when substantially all of the sulfur compound originally charged to the reaction vessel has been consumed. Preferably, the mercaptan/sulfur reaction is substantially complete when the weight of the sulfur compound present in the reaction vessel as a percentage of the weight of the sulfur compound originally charged to the reaction vessel is less than about 20 percent, more preferably less than about 10 percent, and most preferably less than 5 percent. The reaction period can be from about 0.5 to about 20 hours, more preferably from 1 to 10 hours.
The conditions employed during the reaction period typically include elevated temperatures and variable pressures. Stirring of the liquid reaction solution can also be employed to accelerate the mercaptan/sulfur reaction. The temperature employed during the reaction period is preferably from about 50xc2x0 C. to about 150xc2x0 C., more preferably from about 80xc2x0 C. to about 130xc2x0 C., and most preferably from 95xc2x0 C. to 115xc2x0 C.
The reaction period preferably comprises an initial reaction period and a final reaction period. The initial reaction period can commence with the commencement of the mercaptan/sulfur reaction and terminate when the amount of hydrogen sulfide in the liquid reaction solution reaches a xe2x80x9cdisulfide inhibiting hydrogen sulfide concentrationxe2x80x9d. The xe2x80x9cdisulfide inhibiting hydrogen sulfide concentrationxe2x80x9d is preferably such that the weight of liquid hydrogen sulfide as a percentage of the total weight of the liquid reaction solution is from about 0.1 percent to about 10 percent, more preferably from about 0.3 percent to about 5 percent, still more preferably from about 0.5 percent to about 2 percent, and most preferably from 0.8 percent to 1.5 percent.
When the mercaptan and sulfur compound react, hydrogen sulfide evolves. During the initial reaction period, it may be necessary to vent gaseous hydrogen sulfide from the reaction vessel in order to prevent undesirably high pressures within the reaction vessel. However, the method of venting hydrogen sulfide from the reaction vessel preferably employs a venting rate which allows for the concentration of hydrogen sulfide in the liquid reaction solution to increase during the initial reaction period. Preferably, during the initial reaction period, the hydrogen sulfide concentration in the liquid reaction solution increases from essentially zero to the disulfide inhibiting hydrogen sulfide concentration. The increase in hydrogen sulfide concentration in the liquid reaction solution can be caused by adding hydrogen sulfide to the liquid reaction solution from an external source and/or by utilizing the hydrogen sulfide which evolves in-situ, via the mercaptan/sulfur reaction. Preferably, the hydrogen sulfide present in the liquid reaction solution is produced in-situ.
Although the disulfide inhibiting hydrogen sulfide concentration can be reached rapidly by employing a very slow rate of hydrogen sulfide venting during the initial reaction period, it is preferred in practicing the present invention for the rate of hydrogen sulfide venting during the initial reaction period to be such that the initial reaction period is sufficiently long to allow the majority (i.e., more than 50 wt.%) of the sulfur compound originally charged to the reaction vessel to be consumed. Preferably, the initial reaction period is sufficiently long to allow from about 50 weight percent to about 80 weight percent of the sulfur compound originally charged to the reaction vessel to be consumed. Most preferably, the initial reaction period is sufficiently long to allow from about 60 weight percent to about 75 weight percent of the sulfur compound originally charged to the reaction vessel to be consumed.
The reaction pressure during the initial reaction period can vary widely due to the evolving and venting of hydrogen sulfide. Preferably, the reaction pressure during the initial reaction period is from about 1 atmosphere to about 20 atmospheres, most preferably from about 1 atmosphere to about 10 atmospheres.
The initial reaction period can be from about 0.2 hours to about 10 20 hours, preferably from about 0.5 hours to about 8 hours, and most preferably from 1 hour to 3 hours.
After the initial reaction period, the amount of hydrogen sulfide in the liquid reaction solution is preferably maintained at a disulfide inhibiting hydrogen sulfide concentration for a final reaction period. The final reaction period can commence when the amount of hydrogen sulfide in a liquid reaction solution reaches the disulfide inhibiting hydrogen sulfide concentration and terminate when the mercaptan/sulfur reaction is substantially complete. The mercaptan/sulfur reaction is substantially complete when substantially all of the sulfur compound originally charged to the reaction vessel has been consumed. Preferably, the mercaptan/sulfur is substantially complete when the weight of the sulfur compound in the reaction vessel as a percentage of the original weight of the sulfur compound charged to the reaction vessel is less than about 20 percent, more preferably less than about 10 percent, and most preferably less than 5 percent.
The disulfide inhibiting hydrogen sulfide concentration maintained during the final reaction period can be maintained by any method known in the art. Preferably, the disulfide inhibiting hydrogen sulfide concentration is maintained by slowing and/or terminating the release of hydrogen sulfide from the reaction vessel. Most preferably, the disulfide inhibiting hydrogen sulfide concentration is maintained by sealing the reaction vessel during the final reaction period, thereby trapping the hydrogen sulfide in the reaction vessel.
The reaction pressure during the final reaction period is typically from about 1 atmosphere to about 10 atmospheres, more typically from about 1 atmosphere to about 5 atmospheres.
The final reaction period can be from about 0.1 hours to about hours, preferably from about 0.2 hours to about 5 hours, and most preferably from 0.5 hours to 2 hours.
After the final reaction period, the conditions within the reaction vessel are changed from the final reaction conditions to xe2x80x9ctrisulfide enhancing conditionsxe2x80x9d. The transition between final reaction conditions and trisulfide enhancing conditions preferably takes place during a relatively short transition period. Preferably, such transition period is less than 1 hour, more preferably less 0.5 hours, and most preferably less than 0.25 hours.
During the transition period, it is preferred for the temperature within the reaction vessel to be reduced from the final reaction temperature to a xe2x80x9ctrisulfide enhancing temperaturexe2x80x9d. The trisulfide enhancing temperature is preferably less than about 90 percent, more preferably less than about 70 percent, and most preferably less than 60 percent of the final reaction temperature. The trisulfide enhancing temperature is preferably from about 10xc2x0 C. to about 80xc2x0 C., more preferably from about 25 xc2x0 C. to about 65xc2x0 C., and most preferably from 40xc2x0 C. to 50xc2x0 C.
During the transition period, it is preferred for the pressure in the reaction vessel to be reduced from the reaction pressure to a xe2x80x9ctrisulfide enhancing pressurexe2x80x9d. The trisulfide enhancing pressure is preferably less than about 50, more preferably less than about 25 percent, and most preferably less than 10 percent of the final reaction pressure. The trisulfide enhancing pressure can be from about xe2x88x92150 psig to about 25 psig, preferably from about xe2x88x9250 psig to about 10 psig, more preferably the trisulfide enhancing pressure is less than 0 psig, even more preferably from about xe2x88x9210 psig to about xe2x88x921 psig, and most preferably from xe2x88x925 psig to xe2x88x922 psig.
During the transition period, it is preferred for the concentration of hydrogen sulfide within the liquid reaction solution to be decreased from the disulfide inhibiting hydrogen sulfide concentration to a xe2x80x9ctrisulfide enhancing hydrogen sulfide concentrationxe2x80x9d. The trisulfide enhancing hydrogen sulfide concentration is preferably less than about 50 percent, more preferably less than about 20 percent, and most preferably less than 10 percent of the disulfide inhibiting hydrogen sulfide concentration. The trisulfide enhancing hydrogen sulfide concentration is preferably such that the weight of liquid hydrogen sulfide as a percentage of the total weight of all liquid phase compounds in the reaction product is less than about 5 percent, more preferably less than about 2 percent, and still more preferably less than about 1 percent, even more preferably less than about 0.5 percent, and most preferably less than 0.2 percent.
After the transition period, the reaction product is maintained at trisulfide enhancing conditions for a xe2x80x9ctrisulfide enhancing periodxe2x80x9d. The trisulfide enhancing period can commence when trisulfide enhancing conditions are present in the reaction vessel and terminate when the organic polysulfide product has a xe2x80x9cdesired organic polysulfide distributionxe2x80x9d.
The trisulfide enhancing conditions maintained during the trisulfide enhancing period are preferably effective to inhibit the presence of organic disulfides and organic tetrasulfides in the organic polysulfide product. The amount of organic trisulfide in the organic polysulfide product is enhanced by employing conditions which promote the conversion of organic tetrasulfides to organic trisulfides while, at the same time, inhibiting the conversion of organic trisulfides to organic disulfides.
Trisulfide enhancing conditions typically include a low temperature, a low pressure, and a low concentration of hydrogen sulfide in the liquid reaction product. Preferably, the trisulfide enhancing temperature and pressure employed during the trisulfide enhancing period are sufficient to cause substantially all liquid hydrogen sulfide present in the liquid reaction solution to vaporize into gaseous hydrogen sulfide, thereby providing the trisulfide enhancing hydrogen sulfide concentration, described above.
The desired organic polysulfide distribution, which signals the end of the trisulfide enhancing period, is preferably indicated by an organic polysulfide product that has a weight percent of organic trisulfides that is greater than about 80 percent, more preferably greater than about 90 percent, even more preferably greater than about 95 percent, and most preferably greater than 97 percent of the total weight of the organic polysulfide product. The desired organic polysulfide product preferably has a weight ratio of organic trisulfides to organic disulfides which is greater than about 25: 1, preferably greater than about 50:1, more preferably greater than about 80:1, and most preferably greater than 95:1. The desired organic polysulfide product preferably has a weight ratio of organic trisulfides to organic tetrasulfides which is greater than about 25:1, preferably greater than about 50:1, more preferably greater than about 80:1, and most preferably greater than 95:1.
To determine when the liquid reaction solution contains an organic polysulfide product of desired organic polysulfide distribution, the composition of the liquid reaction product can be monitored during the tetrasulfide inhibiting reaction period. The concentration of the liquid reaction solution can be monitored by any method known in the art, for example by sampling and gas chromatograph analysis.
The trisulfide enhancing period can be from about 0.5 hours to about 20 hours, preferably from about 1 hour to about 10 hours, and most preferably from 2 hours to 4 hours.
After the trisulfide enhancing period, the reaction product can be contacted with carbon dioxide to provide a stable polysulfide product. As used herein, the term xe2x80x9cstablexe2x80x9d refers to product that does not substantially turn cloudy or hazy or increase in mercaptan content during the production or storage for at least 30 days, preferably 6 months. The term xe2x80x9csubstantiallyxe2x80x9d means more than trivial.
The amount of carbon dioxide required to produce a stable product can be in the range of from about 0.1 to about 100,000, preferably about 0.5 to about 10,000, and most preferably 1 to 1,000 molar equivalent of the base used in the catalyst. The contacting of the product with carbon dioxide can be carried out under any conditions that are effective to produce a stable organic polysulfide product or can reduce susceptibility of the reaction product to decomposition during heating or sparging.
The residual hydrogen sulfide present in the product after the trisulfide enhancing period can be removed by venting. Removal of the residual hydrogen sulfide can take place either before or after contacting the liquid reaction solution with carbon dioxide to produce a stable product.
After residual hydrogen sulfide has been removed and the product has been stabilized, the unreacted mercaptan can be removed by any means known to one skilled in the art such as, for example, distillation and nitrogen sparging.
Further purification, separation, and recovery methods known to one skilled in the art can be used to recover a substantially pure organic polysulfide product.